Nanometer engineering of metal-support catalysts

ABSTRACT

A method of forming a catalyst body by forming a first layer of hemispherical grain polysilicon over a substrate, and oxidizing at least a portion of the first layer to form a second layer of silica. Additionally, forming a third layer of nitride material over the second layer, and forming a catalyst material over the nitride layer, can be performed before annealing to form a catalyst body.

FIELD OF THE INVENTION

The present invention relates to catalysts, and more particularly, to amethod of fabricating catalysts on a nanometer scale for wide rangingindustrial applicability. The invention also relates to the catalyststructures so formed.

BACKGROUND OF THE INVENTION

Because of its economic importance, catalysis is one of the mostintensely pursued subjects in applied chemistry and chemicalengineering. Catalysts are widely used today to lower the activationenergies that would otherwise prohibit important reactions fromproceeding. Most industrial reactions are catalytic, and many processimprovements thereto result from the discovery of better chemicalroutes, or as the result of attaining new ways to position catalysts tobetter interact with important chemical reactants. Ideally it is oftenbest to expose as much of the catalyst as possible to the reactants inthe reaction scheme.

Unfortunately, to-date, catalysis has been an inexact science on otherthan a macroscopic scale. Scientists are only beginning to understandthe microscopic interplay between the catalyst and the surface orsubstrate upon which it is positioned. It is believed that these surfaceinteractions may have a significant impact on ultimate catalyststereochemistry and performance.

However, because of current process technological limitations it hasbeen difficult to provide reaction vehicles which would facilitatemicroscopic catalyst formation. What is therefore needed in the art is anew method of forming a catalyst body that takes maximum advantage ofthe chemical and physical properties of both the catalyst and thesubstrate upon which it is formed at a nanometer-scale level.

SUMMARY OF THE INVENTION

The invention provides a method of forming a catalyst body on ananometric scale sufficient to carry out microscopic (nanometric)catalysis reactions. The method comprises forming a first layer ofhemispherical grain polysilicon over a substrate. At least a portion ofthe first layer is then oxidized to form a second layer of silica overthe remaining portion of the first layer which has not been oxidized. Athird layer of nitride is formed over the second layer, and a catalystmaterial is deposited on the nitride layer. The catalyst material isthen annealed to form a catalyst body which causes the catalyst materialto have a larger exposed surface area than just prior to annealing.

The invention provides a method for increasing the surface area of acatalyst material. A barrier layer is formed over a layer of silica, anda layer of catalyst material is then deposited over the barrier layer soas to form a catalyst body. The catalyst material incorporated into thecatalyst body has more exposed surface area for reaction than when ithas not been made a part of the catalyst body.

The invention also provides a method of converting a portion ofhemispherical grain polysilicon to silica without substantiallyflattening the textured grain itself The grain is heated to atemperature within the range of about 350 to about 750 degrees C. for aperiod not exceeding about 5 minutes.

The invention provides a catalyst body which has a first layer ofhemispherical grain polysilicon, a second layer of silica formed from aportion of the polysilicon, a third layer of silicon nitride providedover the second layer, and a catalyst material layer provided over thethird layer. The catalyst material layer is made up of at least onemember selected from Group VIII metals and zeolites.

The invention provides a catalyst body having a first layer ofhemispherical grain polysilicon, a second layer of silica formed from aportion of the hemispherical grain polysilicon, and a catalyst materiallayer formed over the second layer.

The invention also provides a sensor device which has a catalyst bodyformed over a substrate. The catalyst body comprises a first layer ofhemispherical grain polysilicon, a second layer of silica formed overthe first layer, a third layer of nitride formed over the silica layer,and a catalyst material layer formed over the nitride layer. It isdesirable that the catalyst material layer be more expansive, e.g.extend further perimeter wise, than at least one of the underlyinglayers.

These and other advantages and features of the present invention willbecome more readily apparent from the following detailed description anddrawings which illustrate various exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three-dimensional exposed view of a hemispherical grainpolysilicon (HSG) substrate formed as part of the process for making acatalyst body according to the method of the invention.

FIG. 1A is a cross sectional side view of the polysilicon shown in FIG.1A along the line 1-1.

FIG. 2 is the catalyst body of FIG. 1A in a further stage ofpreparation.

FIG. 3A is the catalyst body of FIG. 2 in a further stage ofpreparation.

FIG. 3B is the catalyst body of FIG. 2 in a further stage of preparationaccording to another alternative embodiment of the invention.

FIG. 3C is the catalyst body of FIG. 2 in a further stage of preparationaccording to an alternative embodiment of the invention.

FIG. 4A is the catalyst body of FIG. 3A in a further stage ofpreparation.

FIG. 4B is a close-up view of the catalyst body shown in FIG. 4A.

FIG. 4C is a close-up view of the catalyst body shown in FIG. 4B in analternative embodiment of the invention.

FIG. 5A is the catalyst body of FIG. 4A in a further stage ofpreparation.

FIG. 5B is the catalyst body of FIG. 4C in a further stage ofpreparation.

FIG. 6 illustrates an application of the catalyst body formed in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is directed to a method of forming a catalyst body, andalso to the catalyst body produced in accordance therewith.

Referring now to the drawings, in which like numerals indicate likecomponents throughout the various embodiments, FIG. 1 is an exposedthree-dimensional view of a hemispherical grain (HSG) polysilicon layer12 which has been formed on a generic substrate 10. The substrate 10 canbe comprised of silicon, silicon-on-insulator, other semiconductormaterials, or insulator material, and can also be any type of industrialbase or foundation structure. FIG. 1A is an exposed side view of theembodiment shown in FIG. 1. The HSG polysilicon layer 12 ischaracterized by its textured, bumpy surface as illustrated in thedrawings. To form the HSG polysilicon layer 12 shown in the figures,amorphous silicon is first deposited on substrate 10 using one or moreavailable techniques such as chemical vapor deposition (CVD), lowpressure chemical vapor deposition (LPCVD) or rapid thermal chemicalvapor deposition (RTCVD). Preferably, LPCVD is utilized within atemperature range of about 400 to about 600 degrees C., with about 450to about 550 degrees C. being preferred, and at a pressure within therange of about 0.1 to 10 milliTorr, with about 0.5 to 1 milliTorr beingdesirable. Alternatively, the HSG polysilicon layer 12 may be formed asdescribed above, but with the additional step of exposing the surface toa solution of tetramethylammonium hydroxide (TMAH) and hydrogen fluoride(HF) as set forth in U.S. Pat. No. 6,083,849 to Ping et al.,incorporated herein by reference. After formation of the layer 12, theHSG polysilicon will contain individual grains or bumps 14 which areapproximately 50 to 80 nanometers in diameter and about 25 to about 100nanometers high.

Referring now to FIG. 2, once the HSG polysilicon layer 12 has beenformed, at least a portion thereof is converted to a silica (SiO₂) layer16 to provide a support for a subsequently deposited catalyst material.An oxidation process, or more preferably a rapid thermal oxidation (RTO)process, can be utilized for this purpose. In this procedure, the HSGpolysilicon layer 12 is exposed to an oxygen (O₂) plasma in a suitablereaction chamber which is generally available. Oxidation temperaturesare relatively low, and are typically within the range of about 350degrees C. to about 750 degrees C. A temperature range of about 400degrees C. to about 500 degrees C. is more preferred, with a range ofabout 450 degrees C. to about 500 degrees C. being even more desirable.The time for reaction should be relatively short, on the order of nomore than a few minutes. Preferably, the reaction time should be withinthe range of about 1 minute to about 5 minutes. It has now been foundthat shorter reaction times make it possible to attain a silica layer 16which covers at least a portion of, and more preferably most of, andeven more preferably substantially all of the polysilicon layer 12, butone which will not flatten the textured surface of the underlying layer12. It is furthermore desirable to obtain a silica layer which is on theorder of about a few Angstroms thick, e.g. about 1 to about 5 Angstroms,up to about 50 Angstroms thick, and preferably about 20 to about 50Angstroms in thickness.

In a further embodiment of the invention shown in FIGS. 3A and 3B, anitride layer 18 may be formed atop the silica layer 16 using availabletechniques such as rapid thermal nitridation (RTN). For example, ammonia(NH₃) plasma may be presented in a suitable reaction chamber to form asilicon nitride (Si₃N₄) layer 18 over the silica layer 16. Otheravailable nitride materials can also constitute the layer 18. Thus,ammonia diffused with oxygen may be used to form a silicon oxynitridelayer, and this layer would also be encompassed under the more genericterm “nitride layer”.

The nitride layer 18 may also be referred to as a barrier layer whichmay help to prevent any subsequently deposited catalytic metallic ionsfrom penetrating too deeply into the silica layer. In this way, thedeposited catalyst material will be better positioned to anneal into amore efficacious catalyst surface, as hereinafter described. The nitridelayer 18 may be a discontinuous layer as shown in FIG. 3A, or as shownin FIG. 3B may be a continuous layer. It can vary in depth from a fewAngstroms, e.g. about 1 to about 5 Angstroms, up to a few hundredAngstroms, e.g. about 100 to about 500 Angstroms. In an alternativeembodiment of the invention as shown in FIG. 3C, the nitride layer 18may be formed as a continuous layer 18 between the HSG polysilicon layer12 and the silica layer 16, again using the method of nitridation asheretofore described.

As set forth, the nitride layer 18 shown in FIGS. 3A through 3C can, insome embodiments, function as a barrier layer. For example, layer 18 inFIGS. 3A and 3B may serve to prevent unwarranted chemical interactionsbetween a subsequently deposited catalyst material layer (hereinafterdescribed) and the silica layer 16. Thus, the nitride layer 18 may alsosometimes be referred to as a generic barrier layer 18. Other availablechemical compounds which therefore function as barrier material, inaddition to silicon nitride or silicon oxynitride, may also be utilizedas a barrier layer as part of the invention.

Those skilled in the art may discover that the degree of nitridation (oroxynitridation) and thus the extent of the nitride layer 18 (in the x, yand z directions) may be modified to control the crystal orientation ofthe subsequently deposited metal catalyst material. The degree ofnitridation may also be regulated to increase the catalyst surface area,e.g. the extent to which the deposited catalyst material “balls up” onthe surface of the nitride layer, thereby exposing more of its surfacearea.

For purposes of further discussion, the invention will be described withreference to the use of the structure shown in FIG. 3A which has adiscontinuous nitride layer 18 over the silica layer 16. However, it isto be understood that the fabrication steps described below can beapplied to the structures of FIGS. 3B and 3C as well. Referring now toFIGS. 4A and 4B, a catalyst material layer 20 is next deposited over thenitride layer 18 using techniques such as physical vapor deposition(PVD). Any sort of catalyst material is contemplated for use with theinvention, in particular those catalysts based on the transition metalsof Group VIII of the periodic table. For example, suitable non-limitingcatalysts can include iron (Fe), cobalt (Co), nickel (Ni), ruthenium(Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) andplatinum (Pt), as well as their alloys, oxides, salts (e.g. PdCl₂), andmetallic complexes, etc. Of these, ruthenium, cobalt, palladium andplatinum may be preferred in many applications. Other useful catalystscan include zeolites. The deposition process is performed so as to yielda catalyst material layer 20 of about 5 to about 500 Angstroms over thenitride layer 18. The catalyst material layer 20 may be a continuouslayer (if the structure shown in FIG. 3B has been utilized), or as shownin FIGS. 4A and 4B may be a discontinuous layer which is formed overareas containing the nitride layer 18.

The catalyst material layer 20 is deposited so as to be at leastpartially, and more desirably, substantially co-extensive with thenitride layer 18. More preferably, however, the catalyst material layer20 is larger than, e.g. extends outwardly of, the underlying nitridelayer 18 so that an angle θ is formed between an extending segmentportion 20 a of the catalyst material layer 20 and the silica layer 16,as measured from the meeting point 19 shown in FIG. 4B where thecatalyst material 20 contacts the nitride layer 18. The angle θ may bereferred to herein as the wetting angle. It is desirable that there bean extending segment portion 20 a for at least a part of the perimeterof the catalyst material layer 20, and preferably for at least most oreven all of the perimeter of the catalyst material layer. The extendingsegment portion 20 a will preferably be less than about 10 Angstroms inlength as measured from the dotted line 20 b which crosses the endpointof the underlying layer 18 (which in FIG. 4B coincides with the meetingpoint 19). Thus, in a preferred embodiment, the catalyst material layer20, together with its extending segment portion 20 a, can extendoutwardly to be at least about 10% larger than the nitride layer 18.Even more preferably, the layer 20 can extend outwardly to be at leastabout 20% larger than the layer 18.

In an alternative embodiment of the invention shown in FIG. 4C, there isno nitride layer 18 formed, and a catalyst material layer 20 has beenformed over the silica layer 16. There is also shown a wetting angle φwhich is formed between the extending portion 20 a of the catalystmaterial layer 20 and the top of the silica layer 16. In thisembodiment, the line 20 b crosses the meeting point 22 of the catalystmaterial layer 20 and the underlying silica layer 16. The extendingsegment portion 20 a is thus measured from the line 20 b.

The wetting angles θ and φ shown in FIGS. 4B and 4C, respectively, arerepresentative of non-wetting materials generally. The wetting angle θand φ is a thermodynamic variable that depends on the interfacialtensions of the surfaces, and can be less than about 60 degrees for eachalternative embodiment.

Referring now to FIGS. 5A and 5B, after deposition of the catalystmaterial, gas annealing is utilized to convert the amorphous depositedcatalyst material layer 20 into a more ordered mass, preferably a moreordered crystalline mass, which exposes more surface area of thecatalyst material itself than before annealing is performed. Theannealing process is typically conducted at a temperature within therange of about 200 to about 500 degrees C. In one embodiment, thecatalyst material layer 20 may be transformed into a protrudingbump-like bulging structure (when viewed in two dimensions) or abullet-shaped structure (when viewed in three dimensions), as shown inFIGS. 5A and 5B. While FIGS. 5A and 5B show the corners of the catalystmaterial layer 20 to be bulging or rounded, it is also within the scopeof the invention that these corners also be substantially pyramidal aswell. Other shapes of the annealed catalyst material layer 20 which willfacilitate increased exposed surface area of the catalyst are alsowithin the scope of the invention, and thus the shape of the embodimentsshown in FIGS. 5A and 5B should not be construed as limiting. Onceagain, the skilled artisan may discover that the degree of nitridationand the subsequent extent of the nitride layer 18 may be modified to inturn affect the surface area of the deposited catalyst.

It is also preferred that wetting geometry and the extending segmentportion 20 a be substantially preserved after the annealing process.Thus respective wetting angles φ and θ shown in FIGS. 5A and 5Bpreferably remain after annealing, but may also be somewhat larger orsmaller than before the annealing process.

In FIG. 5B, an alternative embodiment of the invention is shown in whichthe catalyst material 20 has been annealed over the silica layer 16,where no nitride layer 18 has been formed (from FIG. 4C). The wettinggeometry and extending segment portion 20 a and wetting angle φ areshown as well. After annealing is complete, the catalyst bodies shown inFIGS. 5A and 5B are then ready for use in various catalyticapplications.

In a further embodiment of the invention, the catalyst material 20 maybe deposited using available masking techniques, for example. The masksmay be patterned to form the catalyst material 20 directly over thenitride layer 18 (FIG. 5A) or the silica layer 16 (FIG. 5B). In thisway, it is possible to eliminate the intervening embodiments shown inFIGS. 4A and 4C, respectively.

One application of the catalyst body formed in accordance with theinvention is in traditional “cracking” operations, in which a catalystis utilized to crack or rupture carbon-carbon (C—C) bonds, usually toconvert a larger molecular weight hydrocarbon into two or more smallerhydrocarbons, e.g. conversion of a paraffin to another paraffin and oneor more olefins or conversion into two or more olefins. The catalystbodies set forth in FIGS. 5A and 5B may be utilized in such reactions,under generally available reaction conditions and parameters, e.g. at atemperature range of about 200 to about 600 degrees C., in a suitablecracking reactor. Myriad other catalyst reactions are available to theskilled artisan, and are also contemplated for use with the catalystbodies of the invention.

FIG. 6 represents a further application of the invention in which asensor 100 is built using catalyst bodies 101 which have been formedaccording to one of the embodiments heretofore described. As can be seenin the figure, a large array of individual catalyst bodies has beenformed on a nanometric scale. In this regard, air which is suspected ofcontaining a material such as for example, a contaminant, can be waftedover the sensor 100. As the suspected material contacts the formedcatalyst material layer 20, catalysis of a reaction can then occurbetween the air-borne material or contaminant and a second reactionmaterial which may present together with the catalyst as in for example,a catalytic metal complex. The reaction between the air-borne materialand the second reaction material may then be detected by any availablemeans, including one or more electrical or optical sensor units 150which may be located above or below the sensor 100, the output of whichmay be fed to an indicator 155. The indicator may be visual or audio innature. The electrical or optical sensor unit 150 may detect a change inelectrical properties, e.g. resistance, or a change in opticalproperties, e.g. reflectance, of the sensor 100 caused by reactions withthe catalyst material. Additionally, the chemical sensor application canutilize the sticking coefficient of the chemical gas to be detected(e.g., hydrogen) to determine the chemical composition of the gas. Thesticking coefficient is a measure of the fraction of incident moleculeswhich adsorb upon the surface, it lies within the range of 0-1, wherethe limits correspond to zero adsorption and total adsorption of allincident molecules respectively. By determining the mass loss to thetotal gas composition, the saturation of chemicals to the sensor surfacecan be detected, and thus the presence of a given chemical can beanalyzed.

As a result of the invention according to one or more of the embodimentsheretofore described, fabrication of several types of catalyst bodies isnow possible in which various structures can be formed with exactingprecision on a nanometric level. These structures are useful in a widevariety of catalytic reactions, and therefore can improve the start ofthe art of catalysis itself.

The foregoing description is illustrative of exemplary embodiments whichachieve the objects, features and advantages of the present invention.It should be apparent that many changes, modifications, substitutionsmay be made to the described embodiments without departing from thespirit or scope of the invention. The invention is not to be consideredas limited by the foregoing description or embodiments, but is onlylimited by the construed scope of the appended claims.

1-47. (canceled)
 48. A catalyst body, comprising: a layer ofhemispherical grain polysilicon; a layer of silica over said layer ofhemispherical silicon grain polysilicon; and a catalyst material oversaid layer of silica.
 49. The catalyst body of claim 48, furthercomprising: a barrier layer between said second layer of silica and saidcatalyst material; and said catalyst material comprising a materialselected from the group consisting of Group VIII metals and zeolites.50. The catalyst body of claim 49, wherein said silica layer is withinthe range of about 5 Angstroms to about 50 Angstroms in thickness. 51.The catalyst body of claim 50, wherein said silica layer is within therange of about 20 Angstroms to about 50 Angstroms in thickness.
 52. Thecatalyst body of claim 49, wherein said barrier layer is a continuousnitride layer.
 53. The catalyst body of claim 52, wherein said barrierlayer is a discontinuous nitride layer.
 54. The catalyst body of claim52, wherein said nitride layer is a silicon nitride layer which isformed to a thickness within the range of about 1 to about 500Angstroms.
 55. The catalyst body of claim 49, wherein said catalystmaterial is at least co-extensive with said barrier layer.
 56. Thecatalyst body of claim 53, wherein said catalyst material has anextending segment portion with respect to an underlying portion of saidnitride layer.
 57. The catalyst body of claim 56, wherein said extendingsegment portion forms an acute angle with respect to said silica layer.58. The catalyst body of claim 53, wherein said catalyst material formsa layer that is at least about 10% more extensive than said nitridelayer.
 59. The catalyst body of claim 58, wherein said catalyst materiallayer is at least about 20% more extensive than said nitride layer. 60.The catalyst body of claim 59, wherein said catalyst material layer isannealed to form a structure having increased surface area.
 61. Thecatalyst body of claim 60, wherein said extending segment portion ispreserved after said annealing.
 62. A sensor device, comprising: acatalyst body formed over a substrate; and a sensor unit coupled to saidcatalyst body and capable of detecting a catalyzed reaction at saidcatalyst body and providing a signal representing occurrence of saidcatalyzed reaction, wherein said catalyst body comprises: a layer ofhemispherical grain polysilicon; and a layer of silica over at least aportion of hemispherical grain polysilicon layer.
 63. The device ofclaim 62, wherein said catalyst body further comprises a catalystmaterial layer over said layer of silica.
 64. The device of claim 63,wherein said catalyst material layer is more extensive than said layerof silica.
 65. The device of claim 63, further comprising a layer ofsilicon nitride over said silica layer, and wherein said catalystmaterial layer is over said layer of silicon nitride.
 66. The device ofclaim 65, wherein said catalyst material layer is more extensive thansaid layer of silicon nitride.
 67. The device of claim 63, wherein saidcatalyst material comprises a material selected from the groupconsisting of Group VIII metals and zeolites.
 68. The device of claim63, wherein said catalyst material comprises at least one metal selectedfrom the group consisting of rhodium, ruthenium, platinum and palladium.69. The device of claim 64, wherein said catalyst material layer isnon-wetting over said silica layer.
 70. The device of claim 66, whereinsaid catalyst material layer is non-wetting over said silica layer. 71.The device of claim 63, wherein said device has sites in which catalystmaterial layer is bump-like in shape.
 72. The device of claim 63,wherein said silica layer has a thickness which is within the range ofabout 20 Angstroms to about 50 Angstroms. 73-81. (canceled)
 82. Acatalyst structure, comprising: a hemispherical grain polysilicon layerover a substrate, said hemispherical grain polysilicon layer having atextured surface; a silica layer over said hemispherical silicon grainpolysilicon layer, wherein said silica layer substantially covers thetextured surface of the hemispherical grain polysilicon layer andsubstantially contours to the textured surface; a plurality of barrierlayer caps over the contours of said silica layer; and a catalystprotrusion over each said barrier layer caps, said catalyst protrusionbeing at least co-extensive with each said barrier layer caps.
 83. Thecatalyst structure of claim 82, wherein said catalyst protrusioncomprises a material selected from the group consisting of Group VIIImetals and zeolites.
 84. The catalyst structure of claim 82, whereinsaid silica layer is between about 1 Angstrom to about 50 Angstromsthick.
 85. The catalyst structure of claim 82, wherein said plurality ofbarrier layer caps comprise a nitride.
 86. The catalyst structure ofclaim 82, wherein said catalyst protrusion is at least 10% larger withrespect to the underlying portion of the respective said nitride layercap.
 87. The catalyst structure of claim 82, wherein said catalystprotrusion is crystalline in structure.
 88. The catalyst structure ofclaim 82, wherein said catalyst body is part of a cracking apparatus.89. The catalyst structure of claim 82, wherein said catalyst body ispart of a sensor apparatus.